The effect of metformin therapy on serum thyrotropin and free thyroxine concentrations in patients with type 2 diabetes: a meta-analysis

Type 2 diabetes and thyroid function disorders are two common chronic endocrine disorders with the high prevalence in various populations. Metformin is well established as the first-line drug therapy for managing diabetes mellitus. In this meta-analysis, we aimed to determine the effect of metformin on serum TSH and FT4 concentrations in patients with type 2 diabetes. We searched PubMed, Scopus, web of science, Cochrane library, and google scholar to collect information on the effect of metformin on serum TSH and FT4 levels. Demographic and clinical information and serum TSH and FT4 concentrations before and after metformin treatment were extracted. Studies on patients over 18 years of age were included. A total of 11 studies including 1147 patients were selected for the final analysis. In hypothyroid patients, the TSH level decreased significantly after treatment with metformin (Hedges’s g:1.55, 95%CI 0.93–2.16, p-value < 0.001); FT4 level increased slightly after taking metformin, but the increase was not significant (Heddges’s g: − 0.30, 95%CI  − 0.90,0.31, p-value = 0.34). In euthyroid subjects, the slight decrease found in TSH and FT4 concentrations was not statistically significant. Metformin reduces TSH levels in hypothyroid patients; however, it has no effect on TSH levels in euthyroid patients. Metformin does not affect serum FT4 levels in euthyroid and hypothyroid patients.


Statistical analysis
Statistical analysis of before/after TSH and FT4 changes (95% CI) was performed with STATA 17.0.We showed each outcome's combined hedges's g, in a random effect model with a forest plot.We evaluated the heterogeneity between the studies with the I 2 index.I 2 values of 0% indicate no heterogeneity, 25% low, 25% to 50% moderate, and 50% high heterogeneity.Publication bias was analyzed by the begg test and presented as a funnel plot.To assess the effect of baseline characteristics of patients, including sample size, mean age, duration of treatment, dose of metformin, and levothyroxine dose, we performed a meta-regression analysis.

Results
After searching the databases and removing duplicate results, we screened 1117 articles based on the title, and 97 articles were selected to review the abstract and text.Finally, 11 studies were evaluated for meta-analysis, including six studies on euthyroid patients, two on hypothyroid patients and three on both euthyroid and hypothyroid patients (Fig. 1).
We assessed bias risks for the studies (Fig. 2).Six studies [10][11][12][13][14][15] with acceptable methods for randomization and three studies 10,11,14 with allocation concealment, rated as low risk of selection bias, while five studies reported insufficient information and were categorized as unclear risk.In addition, we rated seven trials [10][11][12][14][15][16][17] as unclear risk and three studies 13,18,19 as low risk of bias for blinding participants, personnel and investigators. Ten tials had unclear risk of detection bias.We rated nine trials as unclear risk of attrition bias.In addition, we rated five trials 10,13,14,18,19 as having a low risk of reporting bias and three other trials 12,15,16 as having a high risk.

Euthyroid patients
Effect of metformin treatment on TSH Among nine studies on euthyroid patients; seven 10,11,13,16,[18][19][20] were clinical trial, and two 14,15 had retrospective design.Duration of metformin use was less than six months in six studies and between 6 and 12 months in three studies.One thousand one hundred forty-seven people in the metformin group were evaluated.The mean age of the subjects was 54.89 ± 9.69 years.The mean serum TSH concentration was 2.38 ± 0.93 and 2.27 ± 0.90 mIU/L before and after intervention, respectively (Table 1).
Pooling data under the random effect model from nine studies indicated no significant difference in TSH levels before and after taking metformin (Hedges's g: 0.10, 95% CI − 0.06, 0.26, p-value = 0.21) (Fig. 3).The subgroup analysis showed no statistically significant difference based on the duration of treatment and type of study (Fig. 1 Supplementary).

Effect of metformin treatment on FT4
829 people were examined before and after intervention with metformin.The mean age of the subjects was 55.07 ± 7.73 years.The mean FT4 concentration was 11.84 ± 1.87 and 11.79 ± 1.91 ng/dl before and after the intervention, respectively.

Hypothyroid patients
Effect of metformin treatment on TSH Five studies were conducted on hypothyroid patients; four of which were clinical trial 10,12,13,20 and one had a retrospective design 17 .Duration of metformin use was less than six months in three studies and between 6 and 24 months in two studies.Two hundred twelve people in the metformin group were evaluated.The mean age of the subjects was 54.85 ± 5.35 years.The mean TSH concentration was 2.88 ± 0.94 and 1.50 ± 0.84 mIU/L before and after the intervention, respectively (Table 2).Table 1.The characteristics of euthyroid patients in the studies at the baseline and follow-up.#: NR, not reported.*: Mean ± SD.

Discussion
The current meta-analysis revealed that metformin reduces serum TSH levels in patients with hypothyroidism while it has no effect on euthyroid patients' TSH levels.Moreover, metformin did not affect FT4 levels in either hypothyroid or euthyroid individuals.
Metformin's mechanism of action on the HPT axis is complex and multifactorial.Metformin may alter the affinity and/or the number of thyroid hormone receptors, increase central dopaminergic tone, or directly influence TSH regulation, augmenting the effect of thyroid hormones on the pituitary gland 21,22 .It can also increase the absorption of levothyroxine through the gastrointestinal tract, induce subtle changes in the proteins that bind to thyroid hormones, or impact thyroid hormone metabolism 17,20,21 .The central effects of metformin on TRH/TSH regulation may be mediated by the AMPK system, although its exact mechanisms are still unclear 23 .Contrary to its peripheral effects, metformin inhibits AMPK activity in the hypothalamus and increases the inhibitory effect of thyroid hormones on TSH secretion.These effects do not alter the TSH levels in the thyroid Table 2.The characteristics of hypothyroid patients under LT4 monotherapy at the baseline and follow-up.
Another hypothesis is that metformin stored in the body improves thyroid function in both treated and untreated hypothyroid patients.
Serum T4 levels were unaffected by metformin treatment 15,17,20,26 .Due to the lack of evidence regarding increased FT4 levels in metformin users, the hypothesis of a possible effect of metformin on increasing the gastrointestinal absorption of LT4 and resulting suppression of TSH is less relevant.The potential effects of metformin on L-T4 absorption and bioavailability do not appear to influence changes in serum TSH levels.Even though the hypothesis of the effect of metformin on the bioavailability of LT4 drug cannot be ignored, it does not appear convincing to raise this hypothesis given the evidence of the decreasing effect of metformin on TSH in other researches, even in people with subclinical hypothyroidism who do not take levothyroxine.Therefore, there is a possibility that the suppression of TSH in patients is due to a small and continuous increase in thyroid hormones that is not detectable due to an insignificant change.
There are some limitations to the current meta-analysis.First, one study was conducted only on women 17 and there was no information on patient's gender in three studies 12,13,20 .Second, the reviewed studies did not report serum TSH levels less than or greater than 3 mU/l, so that we could not make a comparison in these subgroups.Also, the ratio of T3 to T4 was not investigated in the studies.Finally, we could not classify and report the results in BMI subgroups due to the lack of data on BMI values.
However, future research to examine this topic needs more suitable design with an adequate sample size, and consideration of the effects of different confounding variables e.g.thyroid autoimmunity, BMI, smoking, etc.

Conclusion
Metformin reduces serum TSH levels in hypothyroid patients.However, it has no effect on TSH levels in euthyroid individuals.In addition, metformin does not affect serum FT4 levels in euthyroid or hypothyroid individuals.TSH reducing effect of metformin necessitate clinicians to be more cautious when adjusting LT4 dosage in diabetic patients to avoid hyperthyroidism due to overtreatment especially in the elderly.

Figure 1 .Figure 2 .
Figure 1.Flow diagram demonstrating the study selection for meta-analysis.# Three studies were performed on hypothyroid and euthyroid patients.

Figure 4 .
Figure 4. Forest plot to compare serum FT4 levels before and after metformin therapy in euthyroid patients.

Figure 6 .
Figure 6.Forest plot to compare serum FT4 levels before and after metformin therapy in hypothyroid patients under LT4 monotherapy.
Forest plot to compare serum TSH levels before and after metformin therapy in euthyroid patients.
Forest plot to compare serum TSH levels before and after metformin therapy in hypothyroid patients under LT4 monotherapy.gland with normal function.Nevertheless, based on the evidence regarding TSH reduction in individuals with clinical and subclinical hypothyroidism, it is hypothesized that TSH decreases in individuals with feedback disorders in regulating the HPT axis, but not in those with a normal axis.Other studies have found no change in serum TSH levels in euthyroid people